The detrimental effects of uncorrected astigmatism on visual development are well documented: blur induced by uncorrected astigmatism results in visual deficits that persist despite optical correction. Partnership with the Tohono O'odham Nation, a Native American tribe with a high prevalence of high astigmatism has allowed researchers to conduct large-scale studies of preschool- and school-aged children with high astigmatism. However, little is known about the development of astigmatism and astigmatism- related amblyopia prior to age 3 years. The goals of the present study are: Goal 1. To document developmental changes in astigmatism and aberrations in infants and young children from a population with a high prevalence of high astigmatism. Goal 2. To determine the relation between quality of visual experience (quality of visual input) in astigmatic and non-astigmatic infants and toddlers and visual development (normal and abnormal/amblyopic). Goal 3. To determine the relation between spherical aberration and coma and corrected visual acuity in young children with and without high astigmatism. Goal 4. To determine the effectiveness of eyeglass treatment of astigmatism-related amblyopia in young children. The proposed study will provide unique and valuable information on the development of astigmatism and aberrations and their effects on visual development, will generate important clinical information to aid in the development of guidelines for treatment and prevention of astigmatism-related amblyopia in infancy and early childhood, and will provide a valuable public health service for a population whose children are at increased risk for the development of amblyopia.